<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>开关灯</title>
</head>
<body>
    <button>开关灯</button>
    <script>
        var btn = document.querySelector('button');
        var flag = 0;
        btn.onclick = function() {
            if(flag == 0){
                document.body.style.backgroundColor = 'black';
                flag = 1;
            }else {
                document.body.style.backgroundColor = 'white';
                flag = 0;
            }
        }
    </script>
</body>
</html>
